This invention relates to methods of heating and fixing a toner image, and more specifically it is concerned with a method of heating and fixing a toner image for electrophotographic apparatus wherein a copy sheet on which the toner image is formed is fed between a pair of fixing rolls so as to fix the toner image by fusion adhesion.
The quickest and most positive method for fixing a toner image formed on a copy sheet as by transfer-printing comprises directly heating the toner image by at least one heating roll to cause the toner image to adhere to the copy sheet or supporter by fusion. In one method known in the art, a copy sheet on which a toner image is formed is passed between a pair of heating rolls to bring the toner image into direct contact with high temperature surfaces of the heated heating rolls. The copy sheet is both heated by and pressed between the rolls, this combination of heat and pressure serving to effect the desired fixing of the toner image.
For example, a known form of toner image fixing apparatus comprises an upper fixing roll having an outer layer of material such as silicone rubber and a lower fixing roll fabricated of metal with a smooth surface, disposed to press an advancing image-bearing copy sheet between them. Means such as a coil heater is mounted within the interior of at least one of the rolls for heating the same. The outer surface of the upper fixing roll, which engages the image-bearing surface of the copy sheet, is treated so as to be non-tacky for prevention of offset. Offset, as will be understood, is a condition wherein toner from one copy sheet partially adheres to a fixing roll and is transferred thereby to the next succeeding copy sheet.
In operation, the two fixing rolls are maintained in pressing contact with each other and rotated in opposite directions while a toner-image-bearing copy sheet or a succession of such sheets are advanced between them, one or both rolls being heated by the aforementioned heating means for heating the toner images. However, if the two fixing rolls are maintained in pressing contact when they are not in use (i.e. when they are stationary), the roll having an outer layer of silicone rubber or like material may become permanently deformed; such deformation results in irregularity of pressing contact between the rolls upon rotation thereof, and consequent variation in pressure on a toner image passing therebetween. That is to say, the described deformation of one of the two fixing rolls is undesirable because it adversely affects the fixing of toner images.
In order to obviate this disadvantage, conventional fixing devices of the type described are constructed such that the lower fixing roll is used as a drive roll and the upper fixing roll is moved away from the lower roll when not in use. This makes it necessary to use a mechanism for moving the upper fixing roll and complicates the construction of the device. Besides, this poses the problem of how to adjust the force with which the upper fixing roll bears against the lower fixing roll, because the force with which the upper fixing roll bears against the lower fixing roll immediately after they are brought into pressing engagement with each other naturally varies from the force with which the former bears against the latter when the rolls are heated and their temperature has reached a predetermined fixing temperature (about 180.degree. C). More specifically, thermal expansion of the rolls is not much when the rolls are brought into engagement with each other because the temperature of the rolls is low. However, the rolls will undergo thermal expansion and their diameter will increase as heating progresses. Thus, the force with which the upper fixing roll bears against the lower fixing roll shows a change as heating of the rolls progresses, provided that the pressure applied to the rolls remains constant. This may lead to the development of slip between the copy sheet and the lower fixing roll when the sheet moves between the two fixing rolls because the lower fixing roll is a drive roller. When this happens, the copy sheet will be broken or the image on the sheet will be distorted.
In fixing devices of this type, it is essential that the upper fixing roll and lower fixing roll have the same peripheral velocity at the point of contact between them during the time the toner image on the copy sheet is fused and adheres to the copy sheet.
In order to meet this requirement, it has hitherto been customary in conventional fixing devices of this type to use a one-way clutch or the like to drive both the upper and lower fixing rolls when they are placed in service, so as to prevent the two rolls from moving at different linear speeds at the point of contact of the rolls.
To sum up, conventional fixing devices of this type must use a complex and troublesome mechanism to preclude permanent deformation of the fixing rolls which function as heating rolls.